1. Field of the Invention
This invention relates generally to a material which exhibits thermal bistability and in particular, to a ternary sulfide alloy which exhibits a metal-semiconductor phase transition with hysteresis as a function of temperature.
2. Background of the Related Art
A device incorporating thermally bistable material exhibits hysteresis if the physical properties (e.g., resistivity, optical reflectivity and thermal conductivity) of the material differ over a given temperature range when it is heated versus when it is cooled. Such devices incorporate bistable materials can be used to make a binary switch, whereby the switch is in a first state, the "0" state, when the bistable material is in a first (physical) state, and the switch is in a second state the "1" state when the bistable material is in a second (physical) state. Additional applications of thermally bistable materials are discussed in "Metal-Semiconductor Phase Transitions in Vanadium Oxides and Technical Applications," by F. Chudnovskii, Sov. Phys. Tech. Phys. Vol. 20, p. 999 (1976) and "Optical Properties of Vanadium Dioxide and Vanadium Pentoxide Thin Films," E. Chain, Appl. Optics, Vol. 30, p. 2782 (1991).
Within a device bistable materials have typically undergone a phase charge from a semiconducting state to a metallic state with an increase in temperature. For example, the material can change from (1) a state in which the resistivity is large and decreases as the temperature increases (a semiconducting state) to, (2) a state in which the resistivity is small and increases as the temperature increases (a metallic state). Bistable materials which undergo a phase transition from a semiconducting state to a metallic state with an increase in temperature are said to have a negative temperature coefficient (NTC).
An example of a thermally bistable NTC material with hysteresis is vanadium dioxide (VO.sub.2). In addition to being used in binary switches, vanadium dioxide has been used to make heat pipes and sensor elements. Nevertheless, it is sometimes advantageous that the thermally bistable material have a positive temperature coefficient (PTC) in contrast to a negative temperature coefficient, i.e., the material changes from a metallic phase to a semiconducting phase with increasing temperature as discussed, for example, in "Understanding doped V.sub.2 O.sub.3 as a Functional Positive Temperature Coefficient Material," by B. Hendrix et. al, J. Mater. Sci. Mater. Elect., Vol. 3, p. 113 (1992). These advantages can include low resistivity, high thermal conductivity, and desirable optical properties on the low temperature side of the transition.
It is also desirable to be able to control the temperature at which the bistability in the device occurs. For example, if the device incorporating the bistable material are being used to make temperature sensing switches, it may be necessary that some of these switches undergo a phase transition at a first temperature while others of these switches undergo the phase transition at a second temperature. A useful bistable material should exhibit different transition temperatures with small changes in the chemical composition or with the application of easily accessible pressures.
Quite apart from the study of bistable materials is the study of new barium ternary alloys which may be incorporated in devices. For example, U.S. Pat. No. 2,770,528 discusses barium ferrous group metal ternary sulfides such as barium cobalt sulfide (BaCoS.sub.2) or barium nickel sulfide (BaNiS.sub.2). However, neither of these compounds are thermally bistable materials and consequently, neither BaCoS.sub.2 nor BaNiS.sub.2 exhibits a metallic-semiconducting phase transition such as observed in vanadium dioxide.